Convert all currencies to CRC and poll rates every 6h

Budget/transactions/salarios totals summed Transaction.amount directly,
so USD/EUR entries were treated as CRC and effectively disappeared from
the dashboard (the analytics fix in 9a80f2a only covered analytics).
Adds a shared get_converted_amount_expr() helper driven by the full
Currency enum — USD/EUR via ExchangeRate-API, BTC/XMR via CoinGecko —
and wires it into every func.sum(Transaction.amount) site.

Also starts a background task in the FastAPI lifespan that force-refreshes
every currency 4x/day, persisting USD to the DB and updating in-memory
caches for the rest. Failures are swallowed per-currency so a CoinGecko
outage cannot take out USD/EUR, and the last-known rate is always retained.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/main.py

@@ -1,3 +1,4 @@
+import asyncio
 from contextlib import asynccontextmanager
 
 from fastapi import FastAPI
@@ -7,6 +8,7 @@ from app.api.v1.router import api_router
 from app.config import settings
 from app.db import init_db, run_migrations
 from app.seed import seed_db
+from app.services.exchange_rate import refresh_rates_periodically
 
 
 @asynccontextmanager
@@ -14,7 +16,15 @@ async def lifespan(app: FastAPI):
     init_db()
     run_migrations()
     seed_db()
-    yield
+    rate_refresh_task = asyncio.create_task(refresh_rates_periodically())
+    try:
+        yield
+    finally:
+        rate_refresh_task.cancel()
+        try:
+            await rate_refresh_task
+        except asyncio.CancelledError:
+            pass
 
 
 app = FastAPI(title="WealthySmart API", version="0.1.0", lifespan=lifespan)